Since silicon nitride has outstanding properties in heat resistance, high strength, wear resistance, thermal shock resistance and the like, it attracts attention as engineering ceramics and it has been in practical use for various roll materials such as bearing members and rolling; vanes for compressor, turbo rotors, cutting instruments and the like.
Since the silicon nitride is sintering resistant materials, various additives as well as sintering additives are used for forming sintered bodies. Silicon nitride-rare-earth oxide-aluminum oxide systems, silicon nitride-rare-earth oxide-aluminum oxide-titanium oxide systems and the like are known as the main component systems. In these components, such sintered additives as rare-earth oxide are components for generating grain boundary phase (glass phase) comprising Si—R—Al—O—N compounds (R represents rare-earth elements) and the like during sintering and for developing high strength by densification of sintered bodies.
In the above sintered silicon nitride, addition of sintered additives is a contributing factor for practical application of the material by increasing bending strength, fracture toughness, thermal shock resistance, wear resistance and the like thereof. However, since the material is essentially electric insulator, the material results in sources of various troubles by generating static electricity during their use, causing accretion of fine powders and injuring the accompanied metal. Therefore, development of conductive silicon nitride ceramics is strongly required.
Under the above purpose, there are some examples of research investigating the addition of titanium diboride (TiB2) and zirconium diboride (ZrB2) to silicon nitride-rare earth oxide-aluminum oxide systems. However, the above case requires addition of fairly large amount of electric conductors, leads to lose the essential properties of silicon nitride and is not preferable.
Furthermore, addition of conductive carbon has been examined. Since addition of carbon powders requires fairly large amount of carbon, it greatly inhibits densification of a silicon nitride-sintered additive system and makes it very difficult to produce a compact material (Reference 3). Moreover, there is another example, in which carbon fiber (CNT) is added to silicon nitride-rare-earth oxide-aluminum oxide systems. However, densification is more difficult than the use of carbon powders (Reference 1 and 4).
On the other hand, it is known that addition of titanium oxide, hafnium oxide, zirkonium oxide or the like to a silicon nitride-rare-earth oxide-aluminum oxide system increases wear resistance and addition of aluminum nitride as needed also greatly increases sinterability (Reference 2).
Reference 1: U.S. Patent Publication No. 20040029706
Reference 2: Japanese Patent Application Public Disclosure No. 2004-2067
Reference 3: Cs. Balazsi et al., “Manufacture and examination of C/SiN4 nanocomposites”, Journal of the European Ceramic Society 2004, vol. 24, p 3287-3294.
Reference 4: Material Science and Engineering C23 (2003) 1133-1137.